1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to the transmission of acknowledgment signals in the uplink of a communication system that are generated in response to the reception of multiple scheduling assignments.
2. Description of the Related Art
A communication system consists of a DownLink (DL), conveying transmissions of signals from a base station (also known as “Node B”) to User Equipment (UEs), and of an UpLink (UL), conveying transmissions of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, or the like. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or the like.
The UL of the communication system supports transmissions of data signals carrying the information content, control signals providing information associated with the transmission of data signals in the DL of the communication system, and Reference Signals (RS) which are also known as pilot signals. The DL also supports transmissions of data signals, control signals, and RS. UL data signals are conveyed through the Physical Uplink Shared CHannel (PUSCH). DL data channels are conveyed through the Physical Downlink Shared CHannel (PDSCH). In the absence of PUSCH transmissions, a UE conveys Uplink Control Information (UCI) through the Physical Uplink Control CHannel (PUCCH), otherwise, UCI may be conveyed together with data in the PUSCH. DL control signals may be broadcast or UE related. UE-specific control channels can be used, among other purposes, to provide to UEs Scheduling Assignments (SAs) for PDSCH reception (DL SAs) or PUSCH transmission (UL SAs). The SAs are transmitted from the Node B to respective UEs using Downlink Control Information (DCI) formats through respective Physical Downlink Control CHannels (PDCCHs).
UL control signals include acknowledgement signals associated with the application of a Hybrid Automatic Repeat reQuest (HARQ) process and are typically in response to the correct, or incorrect, reception of the data Transport Blocks (TBs) conveyed in the PDSCH. FIG. 1 illustrates a PUCCH structure for HARQ ACKnowledgement (HARQ-ACK) signal transmission in a Transmission Time Interval (TTI), which in this example consists of one sub-frame. The sub-frame 110 includes two slots. Each slot 120 includes NsymbUL symbols for the transmission of HARQ-ACK signals 130 or for Reference Signals (RS) 140 which enable coherent demodulation of the HARQ-ACK signals. Each symbol further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The transmission in the first slot may be at a different part of the operating BandWidth (BW) than in the second slot in order to provide frequency diversity. The operating BW is assumed to consist of frequency resource units which will be referred to as Resource Blocks (RBs). Each RB is assumed to consist of NscRB sub-carriers, or Resource Elements (REs), and a UE transmits HARQ-ACK signals and RS over one RB 150.
FIG. 2 illustrates a structure for the HARQ-ACK signal transmission using a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence in one slot of the PUCCH. The transmission in the other slot is assumed to effectively have the same structure. The HARQ-ACK bits b 210 modulate 220 a CAZAC sequence 230, for example using Binary Phase Shift Keying (BPSK) or Quaternary Phase Shift Keying (QPSK) modulation, which is then transmitted after performing an Inverse Fast Frequency Transform (IFFT) as it is next described. The RS 240 is transmitted through the unmodulated CAZAC sequence.
An example of CAZAC sequences is given by the following Equation (1):
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j2π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                  n                        +                        1                                            2                                                                      )                                      ]                                              Eq        .                                  ⁢                  (          1          )                    
where L is a length of the CAZAC sequence, n is an index of a sequence element, n={0,1,2, . . . , L−1}, and k is a sequence index. If L is a prime integer, there are L−1 distinct sequences which are defined as k ranges in (1,2, . . . , L−1). Assuming that 1 RB includes NscRB=12 REs, CAZAC sequences with even length can be directly generated through computer search for sequences satisfying the CAZAC properties.
FIG. 3 illustrates a transmitter structure for a CAZAC sequence that can be used without modulation as RS or with BPSK or QPSK modulation as HARQ-ACK signal. The frequency-domain version of a computer generated CAZAC sequence is used in Step 310. The first RB and second RB are selected in Step 320, for transmission of the CAZAC sequence in the first slot and in the second slot, in Step 330, an IFFT is performed in Step 340, and a Cyclic Shift (CS), as it is subsequently described, is applied to the output in Step 350. Finally, the CP is inserted in Step 360 and filtering through time windowing is applied to the transmitted signal 380. A UE is assumed to apply zero padding in REs that are not used for its signal transmission and in guard REs (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not shown.
FIG. 4 illustrates a receiver structure for the HARQ-ACK signal reception. An antenna receives the RF analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal 410 is filtered in Step 420 and the CP is removed in Step 430. Subsequently, the CS is restored in Step 440, a Fast Fourier Transform (FFT) is applied in Step 450, the first RB and the second RB of the signal transmission in Step 460 in the first slot and in the second slot, are selected in Step 465, and the signal is correlated in Step 470 with the replica of the CAZAC sequence in Step 480. The output 490 can then be passed to a channel estimation unit, such as a time-frequency interpolator, in case of the RS, or to a detection unit for the transmitted HARQ-ACK signal.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences and can therefore be allocated to different UEs for HARQ-ACK signal transmission in the same RB and achieve orthogonal UE multiplexing. This principle is illustrated in FIG. 5. In order for the multiple CAZAC sequences 510, 530, 550, 570 generated correspondingly from the multiple CSs 520, 540, 560, 580 of the same root CAZAC sequence to be orthogonal, the CS value Δ 590 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If TS is the symbol duration, the number of such CSs is equal to the mathematical floor of the ratio TS/D the number of such CSs is └TS/D┘ where the └ ┘ (floor) function rounds a number to its lower integer.
In addition to orthogonal multiplexing of different HARQ-ACK signals in the same RB using different CS of a CAZAC sequence, orthogonal multiplexing can also be achieved in the time domain using Orthogonal Covering Codes (OCC). For example, in FIG. 2, the HARQ-ACK signal can be modulated by a length-4 OCC, such as a Walsh-Hadamard (WH) OCC, while the
RS can be modulated by a length-3 OCC, such as a DFT OCC (not shown). In this manner, the multiplexing capacity is increased by a factor of 3 (determined by the OCC with the smaller length). The sets of WH OCCs, {W0, W1, W2, W3}, and DFT OCCs, {D0, D1, D2}, are:
            [                                                  W              0                                                                          W              1                                                                          W              2                                                                          W              3                                          ]        =          [                                    1                                1                                1                                1                                                1                                              -              1                                            1                                              -              1                                                            1                                1                                              -              1                                                          -              1                                                            1                                              -              1                                                          -              1                                            1                              ]        ,          ⁢            [                                                  D              0                                                                          D              1                                                                          D              2                                          ]        =                  [                                            1                                      1                                      1                                                          1                                                      ⅇ                                                      -                    j2π                                    /                  3                                                                                    ⅇ                                                      -                    j4π                                    /                  3                                                                                        1                                                      ⅇ                                                      -                    j4π                                    /                  3                                                                                    ⅇ                                                      -                    j2π                                    /                  3                                                                    ]            .      
Table 1 below presents an example for the mapping for the PUCCH resource nPUCCH used for a HARQ-ACK signal transmission to an OCC nOCC and a CS α assuming a total of 12 CS per symbol for the CAZAC sequence.
TABLE 1HARQ-ACK Resource Mapping to OCC and CSOC for HARQ-ACK and for RSCSW0, D0W1, D1W3, D20nPUCCH = 0nPUCCH = 121nPUCCH = 62nPUCCH = 1nPUCCH = 133nPUCCH = 74nPUCCH = 2nPUCCH = 145nPUCCH = 86nPUCCH = 3nPUCCH = 157nPUCCH = 98nPUCCH = 4nPUCCH = 169 nPUCCH = 1010nPUCCH = 5nPUCCH = 1711 nPUCCH = 11
The SAs are transmitted in elementary units which are referred to as Control Channel Elements (CCEs). Each CCE consists of a number of REs and the UEs are informed of the total number of CCEs, NCCE, in a DL sub-frame through the transmission of a Physical Control Format Indicator CHannel (PCFICH) by the Node B. For a Frequency Division Duplex (FDD) system, the UE determines nPUCCH from the first CCE, nCCE, of the DL SA with the addition of an offset NPUCCH the Node B configures to the UE by higher layers (such as the Radio Resource Control (RRC) layer) and nPUCCH=nCCE+NPUCCH. For a Time Division Duplex (TDD) system, the determination of nPUCCH is more involved but the same mapping principle using the CCEs of the DL SA applies.
FIG. 6 further illustrates the transmission of an SA using CCEs. After channel coding and rate matching of the SA information bits (not shown), the encoded SA bits are mapped to CCEs in the logical domain. The first 4 CCEs, CCE1 601, CCE2 602, CCE3 603, and CCE4 604 are used for the SA transmission to UE1. The next 2 CCEs, CCE5 611 and CCE6 612, are used for the SA transmission to UE2. The next 2 CCEs, CCE7 621 and CCE8 622, are used for the SA transmission to UE3. Finally, the last CCE, CCE9 631, is used for the SA transmission to UE4. After further processing which can include bit-scrambling, modulation, interleaving, and mapping to REs 640, each SA is transmitted in the PDCCH region of the DL sub-frame 650. At the UE receiver, the reverse operations are performed (not shown for brevity) and if the SA is correctly decoded (as determined by the UE through a Cyclic Redundancy Check (CRC) which is masked with the UE identity), the UE proceeds to receive the associated PDSCH (DL SA) or to transmit the associated PUSCH (UL SA).
A one-to-one mapping exists between the resources for HARQ-ACK signal transmission and the CCEs used for the DL SA transmission. For example, if a single resource is used for HARQ-ACK signal transmission, it may correspond to the CCE with the lowest index for the respective DL SA. Then, UE1, UE2, UE3, and UE4 use respectively PUCCH resource 1, 5, 7, and 9 for their HARQ-ACK signal transmission. Alternatively, if multiple CCEs are used for a DL SA transmission, HARQ-ACK information may not only be conveyed by the modulated HARQ-ACK signal but it may also be conveyed by the selected resource (corresponding to one of the multiple CCEs used to convey the DL SA). If all resources within a PUCCH RB are used, the resources in the immediately next RB can be used.
In order to support data rates higher than the ones possible in legacy FDD communication systems operating with a single Component Carrier (CC), BWs larger than the ones of a CC for legacy communications may be used. These larger BWs can be achieved through the aggregation of multiple CCs. For example, a BW of 100 MHz results from the aggregation of five 20 MHz CCs. The Node B can configure communication with a UE over multiple CCs. The PDSCH reception by a UE in each DL CC is configured by a respective DL SA as described in FIG. 6. In TDD systems, higher data rates either in the DL or in the UL can be achieved by allocating a larger number of sub-frames to the specific link. Similar to the aggregation of multiple CCs, in case of multiple DL sub-frames, PDSCH reception in each DL sub-frame is configured by a respective DL SA.
The transmission of HARQ-ACK signals associated with DL SA receptions by a UE in multiple DL CCs can be in the PUCCH of a single UL CC which will be referred to as “primary” UL CC for the UE (the primary UL CC is UE-specific). Separate resources in the primary UL CC can be RRC-configured to UEs for the transmission of HARQ-ACK signals in response to DL receptions in multiple DL CCs.
FIG. 7 illustrates the HARQ-ACK signal transmissions corresponding to DL SA receptions in 3 DL CCs, DL CC1 710, DL CC2 720, and DL CC3 730, that occur in the primary UL CC 740. The resources for the HARQ-ACK signal transmissions corresponding to DL SA receptions in DL CC1, DL CC2, and DL CC3 are respectively in a first set 750, second set 760, and third set 770 of PUCCH resources.
A first approach for a UE to transmit HARQ-ACK signals in response to DL SA receptions in N>1 DL CCs is to simultaneously transmit in N>1 HARQ-ACK channels in the respective resources of the primary UL CC. A second approach is to select the resource used for the HARQ-ACK signal transmission depending on the value of the transmitted HARQ-ACK bits in addition to transmitting a modulated HARQ-ACK signal, as in 3GPP Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE) TDD. In both cases, separate resources for the HARQ-ACK signal transmission are needed in response to DL SA reception for each DL CC. A third approach is to jointly code all HARQ-ACK bits and transmit a single HARQ-ACK signal in an exclusive RRC-configured resource for each UE.
For the transmission of HARQ-ACK signals in the primary UL CC, if the provisioned resources correspond to all CCEs used for SA transmissions in each DL CC, the resulting overhead can be substantial as many DL CCs may exist. A UE receiving SAs in a subset of the DL CCs may not know the number of CCEs used in other DL CCs and therefore cannot know the number of respective HARQ-ACK resources in a sub-frame. As a consequence, the maximum number for the HARQ-ACK resources, corresponding to the maximum number of CCEs in each DL CC, needs to be assumed. If less than the maximum HARQ-ACK resources are used in a sub-frame, the remaining ones cannot usually be assigned to other UL transmissions, such as PUSCH transmissions, resulting to BW waste.
As the number of UEs with reception of DL SAs for multiple DL CCs per sub-frame is typically not large, a pool of resources can be configured by RRC for HARQ-ACK signal transmissions. The resource for HARQ-ACK signal transmission in response to the DL SA reception for the DL CC linked to the primary UL CC can still be determined from the CCE with the lowest index for the respective DL SA. The link between a DL CC and an UL CC is in the conventional sense of a single-cell communication system. Assigning to each UE through RRC signaling unique resources for HARQ-ACK signal transmissions avoids resource collision but it results to resource waste if the UE does not have any DL SA reception in a sub-frame. Assigning to a UE through RRC signaling shared resources with other UEs for HARQ-ACK signal transmissions reduces the probability of resource waste at the expense of scheduler restrictions as UEs with shared resources for HARQ-ACK signal transmissions cannot receive respective DL SAs in the same sub-frame.
The previous considerations apply regardless of the specific method used for the HARQ-ACK signal transmission in the PUCCH or the respective resource determination if one or more PUCCH resources need be reserved for each UE while only a fraction of these resources is typically used in each sub-frame.
Therefore, there is a need to reduce the resource overhead for HARQ-ACK signal transmissions in a primary UL CC.
There is also a need to avoid collisions among resources for HARQ-ACK signal transmissions from multiple UEs.
Finally, there is a need to determine rules for assigning resources for HARQ-ACK signal transmissions to a UE.